The invention relates to a combined GPS/GLONASS satellite positioning system receiver.
The GPS is a system of satellite signal transmitters, with receivers located on the Earth's surface or adjacent to the Earth's surface, that transmits information from which an observer's present location and/or the time of observation can be determined.
The GPS is part of a satellite-based navigation system developed by the United States Defense Department under its NAVSTAR satellite program. A fully operational GPS includes up to 24 Earth orbiting satellites approximately uniformly dispersed around six circular orbits with four satellites each, the orbits being inclined at an angle of 55.degree. relative to the equator and being separated from each other by multiples of 60.degree. longitude. The orbits have radii of 26,560 kilometers and are approximately circular. The orbits are non-geosynchronous, with 0.5 sidereal day (11.967 hours) orbital time intervals, so that the satellites move with time relative to the Earth below. Theoretically, four or more GPS satellites will be visible from most points on the Earth's surface, and visual access to four or more such satellites can be used to determine an observer's position anywhere on the Earth's surface, 24 hours per day. Each satellite carries a cesium or rubidium atomic clock to provide timing information for the signals transmitted by the satellites. Internal clock correction is provided for each satellite clock.
Each GPS satellite transmits two spread spectrum, L-band carrier signals: an L1.sub.GPS signal having a frequency f1=1575.42 MHz and an L2.sub.GPS signal having a frequency f2=1227.6 MHz. These two frequencies are integral multiplies f1=1540 f0 and f2=1200 f0 of a base frequency f0=1.023 MHz. The L1.sub.GPS signal from each satellite is binary phase shift key (BPSK) modulated by two pseudo-random noise (PRN) codes in phase quadrature, designated as the C/A-code and P-code. The L2.sub.GPS signal from each satellite is BPSK modulated by only the P-code. The nature of these PRN codes is described below.
One motivation for use of two carrier signals L1.sub.GPS and L2.sub.GPS is to allow partial compensation for propagation delay of such a signal through the ionosphere, which delay varies approximately as the inverse square of signal frequency f (delay.about.f.sup.-2). This phenomenon is discussed by MacDoran in U.S. Pat. No. 4,463,357, which discussion is incorporated by reference herein. When transit time delay through the ionosphere is determined, a phase delay associated with a given carrier signal can also be determined. The phase delay which is proportional to the time difference of arrival of the modulated signals is measured in real time by cross correlating two coherently modulated signals transmitted at different frequencies L1.sub.GPS and L2.sub.GPS from the spacecraft to the receiver using a cross correlator. A variable delay is adjusted relative to a fixed delay in the respective channels L1.sub.GPS and L2.sub.GPS to produce a maximum at the cross correlator output. The difference in delay required to produce this maximum is a measure of the columnar electron content of the ionosphere.
Use of the PRN codes allows use of a plurality of GPS satellite signals for determining an observer's position and for providing the navigation information. A signal transmitted by a particular GPS satellite is selected by generating and matching, or correlating, the PRN code for that particular satellite. Some of the PRN codes are known and are generated or stored in GPS satellite signal receivers carried by ground observers. Some of the PRN codes are unknown.
A first known PRN code for each GPS satellite, sometimes referred to as a precision code or P-code, is a relatively long, fine-grained code having an associated clock or chip rate of 10 f0=10.23 MHz. A second known PRN code for each GPS satellite, sometimes referred to as a clear/acquisition code or C/A-code, is intended to facilitate rapid satellite signal acquisition and hand-over to the P-code and is a relatively short, coarser-grained code having a clock or chip rate of f0=1.023 MHz. The C/A -code for any GPS satellite has a length of 1023 chips or time increments before this code repeats. The full P-code has a length of 259 days, with each satellite transmitting a unique portion of the full P-code. The portion of P-code used for a given GPS satellite has a length of precisely one week (7.000 days) before this code portion repeats. Accepted methods for generating the C/A-code and P-code are set forth in the document GPS Interface Control Document ICD-GPS-200, published by Rockwell International Corporation, Satellite Systems Division, Revision B-PR, 3 Jul. 1991, which is incorporated by reference herein.
The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite (which includes a complete information about the transmitting satellite within next several hours of transmission) and an almanac for all GPS satellites (which includes a less detailed information about all other satellites). The satellite information transmitted by the transmitting GPS has the parameters providing corrections for ionospheric signal propagation delays suitable for single frequency receivers and for an offset time between satellite clock time and true GPS time. The navigational information is transmitted at a rate of 50 Baud. A useful discussion of the GPS and techniques for obtaining position information from the satellite signals is found in The NAVSTAR Global Positioning System, Tom Logsdon, Van Nostrand Reinhold, N.Y., 1992, pp. 17-90.
The Global Orbiting Navigation Satellite System (GLONASS) has been placed in orbit by the former Soviet Union and now is maintained by the Russian Republic. The GLONASS system also uses 24 satellites, distributed approximately uniformly in three orbital planes of eight satellites each. Each orbital plane has a nominal inclination of 64.8.degree. relative to the equator, and the three orbital planes are separated from each other by multiples of 120.degree. longitude. The GLONASS circular orbits have smaller radii, about 25,510 kilometers, and a satellite period of revolution of 8/17 of a sidereal day (11.26 hours). A GLONASS satellite and a GPS satellite will thus complete 17 and 16 revolutions, respectively, around the Earth every 8 days. The GLONASS system uses two carrier signals L1.sub.GLONASS and L2.sub.GLONASS with frequencies of f1=(1.602+9k/16) GHz and f2=(1.246+7k/16) GHz, where k (=1, 2, . . . 24) is the channel or satellite number. These frequencies lie in two bands at 1.597-1.617 GHz (L1.sub.GLONASS) and 1,240-1,260 GHz (L2.sub.GLONASS). The L1.sub.GLONASS code is modulated by a C/A- code (chip rate=0.511 MHz) and by a P-code (chip rate=5.11 MHz). The L2.sub.GLONASS code is presently modulated only by the P-code. The GLONASS satellites also transmit navigational data at a rate of 50 Baud. Because the channel frequencies are distinguishable from each other, the P-code is the same, and the C/A-code is the same, for each GLONASS satellite.
Both the GPS System and the Global Orbiting Navigation Satellite System (GLONASS) use transmission of coded radio signals, with the structure described above, from a plurality of Earth-orbiting satellites. A single antenna can receive both GPS and GLONASS signals and pass these signals to a signal Receiver/Processor, which (1) identifies the satellite source for each satellite signal, (2) determines the time at which each identified satellite signal arrives at the antenna, and (3) determines the present location of the satellite source.
The range between the location of the GPS and/or GLONASS satellite and the Receiver is equal to the speed of light c times the time difference between the Receiver's clock and the time indicated by the GPS or GLONASS satellite when it transmitted the relevant phase. However, the Receiver has an inexpensive quartz clock which is not synchronized with respect to the much more stable and precise atomic clocks carried on board the satellites. Consequently, the Receiver actually estimates not the true range to the satellite but only the pseudo-range to each GPS or GLONASS satellite.
Both GPS system and GLONASS systems were originally intended as stand alone systems. The GPS receiver design is disclosed by Charles Trimble in the U.S. Pat. No. 4,754,465 and the GLONASS receiver design is disclosed by Gary Lennen in the U.S. Pat. No. 5,486,834.
Although many applications operate with a Receiver designed to receive either GPS or GLONASS signals, many more applications can benefit substantially from combining the two systems in a single Receiver design. This patent application describes a Receiver design capable of producing and using high accuracy measurements from both GPS and GLONASS satellites simultaneously, thus acquiring the benefits associated with using both systems.
Typical applications which may benefit from use of both systems in the same Receiver are Surveying and Mapping, Aircraft navigation, Car Navigation, Marine Navigation, Land Navigation and Scientific Applications.
The use of both systems provides a high degree of system-wide integrity. If GPS or GLONASS suffers a system-wide failure then the Receiver will continue to operate with the remaining GPS or GLONASS operational systems. When both GPS and GLONASS systems are operational, measurement from each of them can be continually compared with the other one in order to detect the system-wide failures.
The system-wide failure includes not only the satellite failing in some manner, but also includes operating in environments where a heavy radio frequency interference is present. The radio interference affecting one system need not affect the other system because GPS and GLONASS operate in a different frequency band.
Each of GPS and GLONASS systems consist of 24 satellites, totalling 48 satellites with both systems. Theoretically, the number 24 in each system was chosen to provide worldwide continuous coverage because at least four satellites are always above the horizon in each system. In practice however, it has ben discovered that at least four satellites above the horizon is not adequate for many applications. For example, in some applications satellites are obscured by obstacles such as buildings, trees and mountains. Hence, the localized environment can prevent a receiving antenna from observing all 4 GPS satellites which may be above the horizon at the particular time of observation. If this is the case, the 4 additional GLONASS satellites above the horizon may be very useful in obtaining the position fixing. In another example, in the real-time kinematic surveying, five or more satellites are required for the operation of a Receiver even in the unobscured by obstacles environment.
Even if the four satellites are sufficient for the position fixing, providing the additional visible satellites leads to the possibility of selection of the four satellites with the minimum position dilution of precision (PDOP). Therefore, the accuracy of the resulting position, velocity, and time measurements can be improved.
In survey applications, more visible satellites lead to a reduction in the time needed to resolve carrier cycle ambiguities, hence improving the speed and integrity of the result.
The GLONASS system of satellites operate at a higher orbit inclination than GPS satellites (64.degree. for GLONASS, 55.degree. for GPS). This leads to GLONASS having better coverage at higher latitudes, e.g. in the State of Alaska or Northern Europe. A combined GPS/GLONASS receiver would incorporate this advantage.
Another advantage of using a GPS/GLONASS receiver is that GLONASS can become a back up system when the US Government intentionally degrades the GPS system accuracy via Selective Availability (SA). The Russian Government insists that it would not intentionally (or has no resources to) degrade the GLONASS system.